1. Technical Field
The present invention relates to image coding devices and image coding methods which compression-code images and records resulting coded image data on a storage medium such as an optical disc, a magnetic disk, and a flash memory. In particular, the present invention relates to image coding devices, image coding methods, image coding integrated circuits, and image coding programs which compression-code images using an H.264 compression coding method.
2. Background Art
Along with advancement of digital video technology, techniques for compressing digital moving picture data have been developed to treat an increasing amount of data. Such development can be seen in the form of compression techniques specifically designed for moving picture data by taking advantage of characteristics of moving picture data.
In addition, increased processing power of information processing apparatuses such as computers has made possible complex computations involved in compression techniques, resulting in a significant increase in compression rate of moving picture data.
For example, a compression technique employed in satellite and terrestrial digital high-definition broadcasting is a compression method referred to as the Moving Picture Experts Group 2 (MPEG-2). For example, for satellite digital high-definition broadcasting, moving picture data is compressed to approximately one-thirtieth of the original using the MPEG-2.
The MPEG-4 AVC/H.264 compression standard, which is one of moving picture data compression-coding methods standardized after MPEG-2, is said to achieve a compression rate approximately twice higher than that of the MPEG-2.
The MPEG-4 AVC/H.264 is employed also as a moving picture data compression method for Blu-ray, which is a standard of optical discs, and for Advanced Video Codec High Definition (AVCHD), which is a standard for recording high-definition moving picture using a video camera. This shows expectations for wide application of the MPEG-4 AVC/H.264.
However, the MPEG-4 AVC/H.264 involves a significantly increased computation amount compared to MPEG-2, because the MPEG-4 AVC/H.264 achieves a high compression rate by implementing combinations of many compression techniques as disclosed in Non-Patent Reference 1.
Such techniques for compressing moving picture data include motion compensation. In coding video through motion compensation, first a motion vector to predict a motion amount is searched for, and then the motion vector obtained by the searching is used for the motion compensation.
Generally, accuracy of estimation of a motion vector is improved by searching a larger range for a motion vector. However, searching a larger range increases the computation amount, thus resulting in higher power consumption. The range searched for a motion vector is thus often determined by considering computation amount or circuit scale.
When a motion vector is searched for within a limited range, the range is not always set to be large enough for motion of an input image.
For example, the range searched for a motion vector is not large enough in the case where an input image is included in a moving picture captured by zooming or panning of a camera or in the case where images preceding and following the input image shows a significantly large motion. This has problematically reduced accuracy of estimation of a motion vector, resulting in lower image quality and lower coding efficiency.
Patent Reference 1 discloses a method for solving such a problem. In the method, coding efficiency is improved by changing methods of predicting motion compensation depending on.    Non-Patent Reference 1: ITU-T Recommendation H.264    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-145011